On current evidence one can postulate at least four stages in the maturation of T lymphocytes. 1) Commitment of early progenitors to the T lineage in bone marrow and spleen; 2) Conversion to immature thymocytes; 3) Generation of mature (medullary) thymocytes and 4) Divergence of the T lineage to yield three functionally specialized subsets. The last three events are marked by characteristic changes in surface phenotype, involving surface antigens (TL, Thy-1, Ly-1, Ly-2, Qa-1-5), as well as lectin receptor sites. A causal relationship between the above stages has not been firmly established. This question is the central issue in the proposed study (whether or not direct progenitor-successor relationships can be recognized). The following recent advances justify a reexamination of thymocyte maturation: 1) We have produced hybridomas secreting monoclonal antibodies to the major T cell antigens, which will significantly improve the precision of serological analysis. 2) The discovery of Qa antigens has opened new horizons in T cell serology. 3) The development of the prothymocyte to thymocyte induction system and its extension to thymocyte maturation step provides a tool to delineate intrathymic T cell development in vitro. 4) Improved analytical and preparatory cell sorting devices, foremost the FACS, will add to the precision of phenotype analysis, particularly if combined with monoclonal antibodies. The aims of this proposal are to design further in vitro T cell induction systems, to establish sequential relationships between thymocytes and T cell subsets, to determine the site of branch points at which T subsets diverge from one another and to learn whether administration of T cell selective monoclonal antibodies (foremost anti Ly antibodies can alter the balance of T cell subsets in vivo.